diff --git a/main.pdf b/main.pdf index 81040ed..901627d 100644 Binary files a/main.pdf and b/main.pdf differ diff --git a/main.tex b/main.tex index b74fd6c..e166d1f 100644 --- a/main.tex +++ b/main.tex @@ -1719,18 +1719,7 @@ degree of temporal integration~(Section\,\ref{sec:constant_feat}). \subsection{Intensity invariance versus SNR along the model pathway} -% % Establishing the principle trade-off (should maybe come later?): -% The output of a transformation is considered to be intensity-invariant if its -% intensity measure saturates for sufficiently large scales $\sca$, which in turn -% caps the output SNR to a constant value across these $\sca$. Otherwise, the -% output SNR will increase monotonically with $\sca$. The trade-off between -% intensity invariance and SNR refers to the principle that a transformation can -% either improve intensity invariance or maintain SNR --- it cannot do both at -% the same time. This principle is most likely not specific to the two mechanisms -% along the model pathway but rather a general property of transformations that -% equalize between different input intensities. - -% Building a sufficient SNR "buffer": +% Building a sufficiently large SNR "buffer": A stridulating grasshopper generates a song with a specific initial intensity, which is steadily attenuated as the song propagates through the environment~(\bcite{michelsen1978sound}). A listening grasshopper receives a @@ -1742,12 +1731,12 @@ filtering of $\raw(t)$ into $\filt(t)$ likely improves the SNR by attenuating frequencies outside the relevant range of grasshopper songs. The SNR is further improved by the rectification and lowpass filtering of $\filt(t)$ into $\env(t)$. The lower the cutoff frequency $\fc$ of the lowpass filter, the -higher the SNR of $\env(t)$ at a given $\sca$, although $\fc$ must also be -sufficiently high to preserve the amplitude dynamics of the song pattern. -Overall, the first processing steps along the pathway are not designed to -achieve intensity invariance but rather to improve the SNR of the song -representation beyond the initial SNR of $\raw(t)$. +higher the SNR of $\env(t)$ for a given $\sca$, although $\fc$ must also be +sufficiently high to preserve the amplitude dynamics of the song pattern. The +first processing steps along the pathway are hence designed to improve the SNR +of the song representation beyond the initial SNR of $\raw(t)$. +% Dependence of log-HP intensity invariance on sufficient SNR (+implications): The first mechanism of intensity invariance consists of logarithmic compression and adaptation of $\env(t)$ into $\adapt(t)$. In the absence of $\noc(t)$, $\adapt(t)$ is a perfectly intensity-invariant representation of $\soc(t)$. In @@ -1757,28 +1746,36 @@ $\raw(t)$ to $\env(t)$ thus serve to improve the intensity invariance of $\adapt(t)$ by shifting the saturation point towards lower $\sca$. However, this effect is limited --- if the SNR of $\raw(t)$ at the receiver's position does not allow for a sufficiently high SNR of $\env(t)$, $\adapt(t)$ will not -be intensity-invariant. The initial song intensity that the sender can achieve -therefore determines the distance at which $\adapt(t)$ is intensity-invariant -to the receiver. +be intensity-invariant. In this case, the receiver is presumably less likely to +recognize $\raw(t)$ as a conspecific song. The limitation of the intensity +invariance of $\adapt(t)$ by the SNR of $\raw(t)$ might hence at least in parts +be responsible for the limited maximum distance at which song recognition is +possible~(\bcite{lang2000acoustic}) and the selection towards song patterns +that are robust to noise masking~(\bcite{einhaupl2011attractiveness}). -Assuming that intensity invariance of $\adapt(t)$ is required for reliable song -recognition, - - -This might be a reason why robustness to noise masking is an -attractive property of male calling songs~(\bcite{einhaupl2011attractiveness}). - -The saturation level of $\adapt$, -unlike its saturation point, is independent of the SNR of $\env(t)$ because the -influence of $\noc(t)$ is negligible for sufficiently large $\sca$. The output -SNR of $\adapt(t)$ saturates at a comparably low value of around 10. This might -in parts be a consequence of the logarithm, which compresses different higher -intensities but also amplifies lower intensities, including the noise floor. -Both the saturation level and the saturation point of $\adapt(t)$ vary between -different species and individual songs. These differences are likely rooted in -the way in which logarithmic compression acts on the specific distribution of -$\env(t)$, which is determined by $\fc$ as well as the temporal structure and -frequency spectrum of the rectified $\filt(t)$. +% Trading SNR for log-HP intensity invariance (+variability, +general principle): +The SNR of each song representation prior to $\adapt(t)$ increases +monotonically with $\sca$~(excluding $0<\sca\ll1$, noise regime). These +representations maintain and improve the initial SNR of $\raw(t)$ and hence +never achieve intensity invariance. In contrast, the SNR of the +intensity-invariant $\adapt(t)$ never exceeds its saturation level even for +arbitrarily high $\sca$. The saturation level of $\adapt(t)$ varies across +species and songs. This variability is likely rooted in the way in which +logarithmic compression acts on the specific distribution of $\env(t)$, which +depends on the $\fc$ of the lowpass filter as well as the temporal structure +and frequency spectrum of the rectified $\filt(t)$. Overall, $\adapt(t)$ has +never been observed to exceed a SNR of around~10 across all songs. The low SNR +of $\adapt(t)$ partially results from the amplification of smaller values of +$\env(t)$ by the logarithm, which raises the noise floor of $\adapt(t)$. Still, +the reduction in SNR is substantial --- considering that the SNR of preceeding +song representations has been orders of magnitude higher --- but is likely a +necessary price to pay for the intensity invariance of $\adapt(t)$. After all, +a transformation cannot compress a range of different input intensities into a +constant output intensity without sacrificing some of the corresponding input +SNR. Accordingly, the trade-off between intensity invariance and SNR is not +expected to be specific to the particular mechanisms along the pathway but +presumably applies to any transformation that achieves or improves intensity +invariance. Thresholding and temporal averaging renders feature $f_i(t)$ intensity-invariant for sufficiently large $\sca$. The trade-off between